1. Field of the Invention
The present invention relates to a device which delivers information on the rotational state of a rotating shaft, for example, a rotating angle, a rotating speed and so forth, by using a sine-wave encoder.
2. Description of the Prior Art
An encoder is attached to the rotating shaft and so forth of a rotary machine and is used for the computation and control of the rotational position of the rotating shaft, the detection of its rotating speed and so forth.
As conventional encoders, pulse encoders whose output wave forms are pulsed are available, and, in addition to these types, sine-wave encoders whose output wave forms are in the shape of a sine wave are also available. These sine-wave encoders, whose outputs vary analogously, have the feature of enabling not only the discrimination of the interval following each pulse as a continuous quantity but also the detection of the rotating speed of a rotary machine, as a continuous quantity.
One of the methods of speed detection is practiced as follows.
Specifically, if an output signal is represented by .upsilon.=a sin .omega.t, this signal is differentiated to give d.upsilon./dt=a.omega. cos .omega.t. If a.omega. is extracted as the quantity of speed detection, the detected quantity can be obtained as the continuous quantity for the variations in .omega..
Therefore, sine-wave encoders have been utilized as analog sensors which are used for speed detection and position detection in order to control a servomotor. This application of a sine-wave encoder has been known and disclosed, for example, in the specifications of Japanese Patent Laid-Open No. 83061/1984, Japanese Patent Laid-Open No. 83062/1984, Japanese Utility Model Laid-Open No. 88143/1981, Japanese Utility Model Laid-Open No. 58365/1984 and Japanese Patent Publication No. 22963/1984, as well as other.
However, this type of sine-wave encoder sensor which employs a photosensor (a combination of a light emitting diode and a photodiode, or that of a light emitting diode and a phototransistor) is of no practical use since the output voltage of the sensor tends to be varied by approximately 50% because of variations in the quantity of light of the light emitting diode due to the temperature and the time-dependent changes thereof. Thus, it is considered that the correction of the quantity of light is effected for stabilization purposes on the light emitting diode. This method has already been disclosed in the specification of Japanese Patent Publication No. 22963/1984, which was successful in improving the light quantity to a certain extent. In this prior art method a signal detecting section and a light quantity correcting section employ an individual photoconductive element, respectively. The reason for this is that a signal is changed into a sine wave form due to an alternating current, and additionally, the frequency of the signal suffers from variations, thereby making it difficult to detect the light quantity by using a photoconductive element for detecting a signal. Therefore, an additional method was adopted to provide another photoconductive element for the correction of the light quantity, thereby enabling the light quantity to be detected so as to make correction. This method is based on the assumption that the photoconductive elements used in both sections possess the same temperature characteristics and time-dependent variations. However, in fact, each element shows different characteristics. Furthermore, since the overall temperature characteristics reach as high as 0.2%/.degree.C., the element varies in characteristics by as much as 10% when the temperature of the element is varied by 50.degree. C., and it can also be expected to vary by approximately 10% in relation to the time-dependent variations. As described above, although improvements have been made to a certain extent, the results thus provided remain unsatisfactory.
It is to be noted that, when a servomotor operates at several thousand rpm with the resolving power of an encoder being increased to as high as 1000-3000 ppr, the frequency range to be used becomes as wide as from 100 to 150 KHz. If the linearities of the characteristics are to be improved, frequency characteristics up to approximately 1 MHz are required. However, at present, conventional sine-wave encoders merely realize the frequency characteristics of -3 dB (a 29.3% attenuation) at 100 KHz.
As described above, conventional sine-wave encoders involve disadvantages in that their output voltages are greatly varied due to their temperature characteristics, and their linearities are deteriorated due to their frequency characteristics, so that they cannot exhibit satisfactory performance as sensors for use in a servomotor.
However, it has been found from experiments that, although a photodiode and a phototransistor show time-dependent variations, the degree of their variations is smaller than that of the variations of a light emitting diode, so that, if some measures are taken to compensate for the time-dependent variations of the light emitting diode, there is normally no problem in practical terms.